By Ron Cowen
Did last New Year’s Eve seem a trifle tedious? Did your celebration go on a little too long? Maybe that’s because just before midnight Greenwich Mean Time—6:59:59 Eastern Standard Time to be exact—the international authority on timekeeping ordered everyone to wait a second. For the 23rd time since 1972, the International Earth Rotation and Reference System Service added an extra second to the time standard, a worldwide network of some 200 atomic clocks.
The clocks, most of them governed by the ultrasteady vibrations of electrons in cesium atoms, are accurate to a tenth of a billionth of a second a day. However, humankind’s oldest clock—Earth’s rotation—isn’t nearly so precise. Primarily in response to the moon’s tidal pull on the oceans, our planet isn’t turning quite as fast as it used to. To keep Earth time and atomic time in sync, experts have agreed to insert a leap second every few years into the official atomic-based standard, which is called Coordinated Universal Time.